The temperature distribution on interior hole surfaces of drawing dies in HFCVD reaction was analyzed by combining the FEM simulation and related measurements. The analysis showed a parabolic tendency of elevating substrate temperatures and a moderating tendency of corresponding temperature gradients while enlarging hot-filament radius. The optimization of the hot-filament radius for a sized drawing die to achieve a comfortable substrate temperature distribution for depositing superior diamond films was conducted and safely supported by related experiments. SEM was used to observe the morphology of diamond films. The simulation is of practical significance for improving the deposition of diamond films in HFCVD reaction.